The present invention concerns a crack detecting window panel assembly and method for producing same, and in particular, a method for monitoring and interrupting power applied to an electrically heated window glass.
Motor vehicles of recent manufacture are being equipped with electrically heated window panel assemblies, i.e., windshields, for defogging and deicing the windows. These electrically heated windshields are becoming increasingly popular, not only due to the convenience they afford the driver with rapid deicing and defogging, but also from the the enhanced safety they afford in the vehicle's operation. These electrically conductive windshields typically draw 5 to 35 amperes of electric current at 12 to 100 volts during heating depending upon the method and materials used in their construction. With these high levels of electrical current consumption, potential problems due to fractures in the window exist.
One possible problem which arises is the potential for electric shock to an individual coming into contact with a fractured windshield and receiving an electrical shock due to power still being applied to the electrically conductive panel.
Another problem is the possibility of local ignition and flame due to an arcing which may occur when a fracture exists in the electrically conductive panel.
Yet another possible problem is the potential for a motor vehicle accident due to startling the driver with the electric arc from the fractured window and then the driver losing control of the vehicle.
Prior artisans have developed crack detectors which have concentrated on two avenues of methodology for circuit interruption. These methods have involved resistance level detection or continuity loops to indicate when a crack has formed in the windshield. An example of one of the resistance measuring type of systems is shown in U.S. Pat. No. 4,565,919, in which the circuit includes structure for monitoring the resistance of the conductive element and interrupting the power applied thereto when the resistance of the conductive element assumes a value indicative of the element being fractured. In another example of the type in which a continuity loop is used, as shown in U.S. Pat. No. 3,941,975, a sensing circuit which includes a frangible strip is disposed in a predetermined pattern on a panel. When the panel fractures, one or more cracks propagate across the frangible sensing circuit to interrupt the same for deenergizing the electrically conductive panel. Finally, U.S. Pat. No. 3,892,947 discloses an electrically heated, frangible panel especially adapted to prevent electrical shocks after fracture or breakage. An electric current conductive strip is adhered in the marginal band and extends around an electrically conductive coating before contacting an edge thereof. Upon fracture or breakage of any portion of the panel, the conductive strip severs preventing current flow through the coating.